Post-translational regulation of plant immunity

Abstract

Plants have evolved multi-layered molecular defense strategies to protect against pathogens. Plant immune signaling largely relies on post-translational modifications (PTMs) to induce rapid alterations of signaling pathways to achieve a response that is appropriate to the type of pathogen and infection pressure. In host cells, dynamic PTMs have emerged as powerful regulatory mechanisms that cells use to adjust their immune response. PTM is also a virulence strategy used by pathogens to subvert host immunity through the activities of effector proteins secreted into the host cell. Recent studies focusing on deciphering post-translational mechanisms underlying plant immunity have offered an in-depth view of how PTMs facilitate efficient immune responses and have provided a more dynamic and holistic view of plant immunity.

Figure 1. Post-translational signaling mechanisms for pattern-triggered immunity

Cell surface-anchored pattern-recognition receptors (PRRs: FLS2, EFR) detect microbe-associated molecular patterns (MAMPs) and initiate pattern-triggered immunity (PTI). Activation of FLS2 and EFR by bacterial-derived peptides flagellin (flg22) and elongation factor Tu (elf18), stimulates the recruitment of the leucine-rich receptor-like kinase (LRR-RLK) BAK1 and the receptor-like cytoplasmic kinase (RLCK) BIK1, and induces auto- and trans-phosphorylation (red circles) of their cytoplasmic kinase domains. Phosphorylation of PRR-RLK-RLCK complex is attenuated by protein phosphatases (PP2A, PP2C38). MAMP perception triggers the production of reactive oxygen species (ROS) by the NADPH oxidase RBOHD at the plasma membrane, which is activated through phosphorylation by calcium-dependent protein kinases (CPKs) and BIK1. PTI signaling is propagated through mitogen activated protein kinase cascades (MAPKKK, MAPKK, MAPK) resulting in phosphorylation and activation of transcription factors (TFs) that induce transcriptional reprogramming for defense. Pathogens deliver effector proteins into the host cell to suppress PTI. For example, the Pseudomonas syringae effector HopAO1 is a protein tyrosine phosphatase that dephosphorylates PRRs to and inhibits the host immune response.

Figure 2. NPRs are regulators of two opposing hormone-mediated immune responses

NPR1 normally exists as a high molecular weight oligomer in the cytoplasm. In response to salicylic acid (SA) - induced changes in cellular redox, NPR1 monomers are released into the nucleus. At resting state, NPR1, which is phosphorylated (red circles) at Ser55/Ser59, interacts with WRKY, a transcriptional repressor of PR genes. Sumoylated (light blue oval) NPR1, which is phosphorylated at Ser11/Ser15 but dephosphorylated at Ser55/Ser59, interacts with the transcriptional activator TGA3 to promote PR gene expression and establishment of systemic acquired resistance (SAR). NPR3 and NPR4 mediate SA-dependent degradation of NPR1 by the proteasome. NPR3 and NPR4 also promote the SA-enhanced degradation of JAZ transcriptional repressors to induce JA gene expression, resulting in activation of JA synthesis and signaling during early effector-triggered immune (ETI) responses.